1. Field of the Invention
The present invention relates to an imaging or image formation process by nuclear magnetic resonance. The use of this process is more particularly intended in the medical field for representing sections of organs of the human body.
2. Discussion of Background
Nuclear magnetic resonance imaging has mainly been developed as a means of medical diagnosis. It makes it possible to display internal tissue structures with a contrast and resolution of a quality not hitherto achieved with other imaging processes. In order to obtain an image by nuclear magnetic resonance of an organ section with differentiation of the tissue characteristics of the organ, use is made of the property of certain atomic nuclei, such as protons, of orienting their magnetic moment whilst acquiring energy when placed in a main constant magnetic field B.sub.0. A particular zone of an object containing nuclei then has an overall magnetic moment, which can be flipped in accordance with a given orientation, perpendicular or parallel to field B.sub.o, by inducing a resonance by the emission of a radio frequency field perpendicular to the main field.
All the particles which then have a magnetic moment rotating at a so-called Larmor precession speed tend to find again the initial orientation parallel to B.sub.o by emitting a radio frequency signal at the characteristic resonant frequency of B.sub.o and of the nucleus. This signal can be detected by a receiving antenna. The duration of the return to equilibrium of the overall magnetic moment of a considered region and the decrease of the signal are dependent on two important factors, namely the spin-system interaction and the spin-spin interaction of the particles with the surrounding material. These two factors lead to the definition of two relaxation times, called respectively T.sub.1 and T.sub.2. A considered region of an object thus emits a signal, whose intensity is dependent on T.sub.1, T.sub.2, the proton density of the region and the time which has elapsed since radio frequency excitation.
In order to locate a region of the organ, it is necessary to establish the nature of its emission as a function of the local conditions of the magnetic field. These local conditions are imposed in such a way that the frequency and the phase of the emission are characteristic of the location in space of said region of the organ. For this purpose, pulsed magnetic field gradients are superimposed on the main field B.sub.o. These gradients are oriented in directions X, Y and Z in order to define, at all times, the volume elements which resonate at known frequencies. For obtaining a complete picture, the local conditions are imposed in programmed sequences, which are stored in a master computer. These sequences define the application times of the gradients, the excitation times of the nuclei by the radio frequency field pulses and the reading or acquisition times of the image data.
Another factor intervenes to modify the intensity of intercepted signal when the nuclei return to their equilibrium orientation. This other factor depends upon the molecular diffusion or scattering of the medium. The molecular diffusion relates to the displacements undergone by the molecules of a medium as a function of time. The inhomogeneity of the magnetic field in which these molecules are located then has the effect that the magnetic resonance frequency of these molecules changes. Thus, this frequency is linked with the gyromagnetic ratio of these molecules at the intensity of said field. Furthermore, during a magnetic resonance experiment, particularly one with a sequence of spin echoes, the intercepted signal is below the expected intensity.
Thus, the frequency of occurrence of the molecules of the region of the space where the magnetic fields differ has the effect of modifying the relative phases of the contributions made by each of these molecules to the intercepted overall magnetic resonance signal. As the displacements of the molecules are in all directions, the phase dispersion resulting therefrom has the effect that certain contributions are mutually opposed. The intercepted signal is then weaker. This sensitivity loss to a certain extent represents the diffusion characteristic of a medium and a highly diffusing medium is subject to a very rapid decrease in its magnetic resonance signal with the echo time used.
In human organs there are pathological tissues, e.g. angiomas and tumors having often identical standard nuclear magnetic resonance signals. In other words, the images of these organs show the relaxation times T.sub.1 or T.sub.2 and do not make it possible to discriminate these conformations. Thus, the examination of the standard image does not make it possible to make a therapeutic decision. Thus, the aim of the present invention is to propose images where the parameter shown is the molecular diffusion characteristic in the studied tissues, in order to improve their differentation.